1. Field of the Invention
The present invention relates to an image field distinguishing apparatus for distinguishing character/line portions from gradation portions of a character/line/gradation-mixed image read from a document by means of a scanner, etc.
2. Description of the Related Art
In recent years, apparatus, such as a facsimile equipment, document file, digital copying machine, etc., have increasingly been able to handle a document image in the form of a digital signal. By handling images as digital signals, many advantages can be obtained, such as various edit/correction processing, and electronic recording and transmission. Such a digital image processing apparatus has conventionally been used to mainly handle black/white binary images, such as characters and lines. A growing demand has recently been made for an apparatus capable of handling a gradation-mixed image.
Recording an image in the form of a hard copy poses a problem from the standpoint of handling a character/line/gradation-mixed image. As a recording method for obtaining a hard copy, use has often been made of a melt thermal transfer system and electrophotographic system. These recording systems have a capacity to represent a density of two or merely several levels per recording image spot. For this reason, techniques, such as an area modulation method, are employed to represent a gradation image. The area modulation method is used to represent a gradation portion by combining together a plurality of image spots. Typically, a systematic dithering method, or other methods known in the art, are used.
The systematic dithering method is theoretically a recording method for achieving a compatibility between the image resolution and the gradation portion. However, this method has drawbacks in that characters/lines are feebly reproduced at their edges through a document reading system. Also, moire noise is produced in a dot pattern document. The character/line information is a very important part of the document image and, further, dot pattern printing is often used to represent a gradation image or portion in the document image. Therefore, these drawbacks of systematic dithering can cause problems with the document image.
Image field distinguishing processing is known to eliminate these drawbacks and to effect image recording in a manner to achieve the image resolution/gradation compatibility. This is a technique for distinguishing a recording image into, for example, a character/line preference portion and a gradation preference portion. The gradation preference portion may be a photo image or dot pattern. In image field distinguishing processing, recording processing is performed in accordance with the results of the distinction.
JPN PAT APPLN KOKAI PUBLICATION 2-199588 has proposed, as one image field distinguishing system, an image field distinguishing method utilizing a feature wherein the characters are finer and narrower than the gradation images. This method comprises subjecting an input image to binary processing in accordance with its density levels to provide white pixel and black pixel portions and distinguishing a white pixel portion of greater area as a background portion and a black pixel portion of smaller area, bounded by the background portion, as a character/line portion. However, a drawback of this method is that since such a method determines, as the background portion, a relatively broad range white portion corresponding to the input image, there is a possibility that a low density gradation portion (low area percentage portion) will be incorrectly determined to be a background portion. That is, there is a problem that distinguishing accuracy is low for some dot patterns.
Further, from the standpoint of the necessity to perform image field distinguishing processing in real time, the character/line portion has to be determined from image signals obtained through the raster scanning of a document, that is, from image signals sequentially input along a horizontal direction. This poses a problem because narrow lateral lines, that is, narrow lines parallel to the horizontal scanning direction, may not be distinguished from the character/line portion.